Multidrug resistance often arises in human tumors due to overexpression of plasma membrane ATP-binding cassette (ABC) transporter proteins. Recent experiments have indicated that multidrug resistant cells frequently exhibit changes in the profile of lipid species known as sphingolipids along with increases in ABC transporter expression and activity. We have recently discovered that multidrug resistant Saccharomyces cerevisiae cells exhibit a similar molecular linkage between increases in ABC transporter expression and changes in sphingolipid biosynthesis. Work from our lab and others has shown that increases in the activity of the zinc cluster transcription factors Pdr1 p and Pdr3p lead to induction of multidrug resistant ABC transporters like Pdr5p and Yor1p but also the long chain base (LCB) transporter Rsb1p. LCBs are bioactive precursors in sphingolipid biosynthesis in eukaryotic cells. We hypothesize that the coregulation of the ABC transporters and LCB transporter Rsb1p is required for normal modulation and function of the plasma membrane. The Ypk protein kinases in S. cerevisiae are activated by LCBs and we will examine the ability of Ypk2p to control multidrug resistance and gene expression through regulation of Pdr3p. Loss of Pdr5p leads to induction of LCB resistance. Genetic analysis will be performed to identify and analyze components of this signaling pathway connecting loss of this ABC transporter with induction of LCB tolerance. Levels of LCBs will also be analyzed in cells lacking Pdr5p and Yor1p as well as in mutant strains with different levels of Pdr1p/Pdr3p transcriptional activity. This will allow us to determine both the identity of LCBs that are regulated by Pdr pathway function and to uncover sphingolipids that may serve to regulate activity of Pdr1p and/or Pdr3p. Finally, the effect of accumulation of sphingolipid precursors on ABC transporter function will be determined. Transporter activity and trafficking will be measured in strains exposed to high levels of LCBs or containing defects in sphingolipid synthesis. Our work indicates that drug resistant yeast cells co-regulate the biosynthesis of drug transporters with constituents of the membranes in which these transporters will function. This coordinate regulation is important to ensure normal membrane function and may allow new antifungal drugs to be targeted towards its interruption as later stages of this pathway are carried out by enzymes that are not found in mammals.